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CHM 1025
Chapter 2 Part A
1. Atomic theory – developed by
a. Summarized known information about atoms
b. Postulates:
i. .
ii. .
iii. .
iv. .
2. Deductions:
a. Element –
b. Compound –
3. 3 Laws
a. Law of Conservation of Mass
b. Law of Constant Composition (definite composition)
c. Law of Multiple Proportions
4. Atomic Symbols: 1 or 2 letter notations as shorthand to represent element names
a. Always Capital—Small. E.g. Au or Hg
5. Subatomic particles are smaller particles found inside the atom; the three most important in chemical
behaviour are the electron, proton, and neutron.
a. Discovery of subatomic particles and the structure of the atom
i. Electron—the negatively charged subatomic particle was discovered by _____________ in
1897
1. Cathode ray tube (CRT) experiment: saw that the cathode ray was straight in the
absence of a electric field, but that it bent towards the positive plate when a field was
applied regardless of the material used to generate the cathode rays—matter comprises
identical, negatively charged particles
2. Determined the charge-to-mass ratio of the electron
3. He hypothesized the ______________ model where electrons are embedded in a
positively charged atom like raisins in the pudding.
4. The charge on an electron was determined by __________________ in his oil drop
experiment: -1.6022 x 19-19 C per electron.
a. Oil droplets were exposed to radiation; they took on a negative charge
b. Measured the magnitude of an electric field necessary to slow falling drops
and then suspend them in the air.
c. Calculated the mass of an electron: 9.1094 x 10-28 g
ii. The proton, then, must have a positive charge equal to that of the electron (atoms are neutral)
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iii. The nuclear model of the atom was suggested by __________________. In his gold foil
experiment he showed that the majority of the atom was empty space and that most of the mass
in the atom is concentrated in a positively charged center.
1. A beam of positively charged alpha particles (a.k.a. He nuclei) was aimed at a thin
layer of gold. A surrounding zinc-sulfide screen detected the particles. Most went
straight through, but some were deflected
Artist’s rendition of the atom
2. Mass of the proton is 1.6726 x 10-24 g—about 1840 times greater than the mass of the
electron
iv. Rutherford postulated the neutral neutron to make up the missing mass of the atom; in 1932
____________________ detected the neutron.
1. The mass of the neutron is 1.6749 x 10-24 g
6. Isotopes, Atomic numbers, and Mass numbers
a. The number of ____________ in the nucleus of an atom is the atomic number (Z). If Z changes, the
identity of the element changed.
b. A neutral atom has the same number of electrons and protons.
c. The mass number (A) is the sum of ____________________ in the nucleus.
Nuclide or isotope symbol
A
Z
C
12
6
C
35
17
Cl
d. An element can have different numbers of neutrons (N), i.e. same Z, but different A. These are called
isotopes.
i. A = Z + N
ii. Examples:
iii. Other ways to write isotope symbols: ______ can be written carbon-14, C-14, or 14C.
e. The only stable isotope of naturally occurring fluorine has a mass number of 19. How many protons,
neutrons, and electrons are in an atom of fluorine? Write the isotope symbol.
f.
Some isotopes have special names:
i. 1H
ii. 2H (D)
iii. 3H
iv. D2O or deuterated water is chemically similar to water, but has different physical properties—
heavy water ice cubes sink in water, but normal ones float.
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